Numerical investigation of building gap effects on traffic pollutant dispersion in urban networks with intersecting streets

Wind passing through the gaps between street-facing buildings has the effects of accelerating the natural removal of air pollutants in street canyons, with wider gap conventionally leading to better air quality. However, previous studies oversimplify the canyons by modeling them as isolated systems with only two rows of buildings, neglecting the blocking effects of surrounding building complexes. This leads to an overestimation of the gap flow's ability to flush out traffic pollutants. In this study, to better represent real urban configurations, a typical network with two crossing streets bounded by four complexes is investigated numerically, where one of the streets is parallel and another is perpendicular to the incoming wind. The gap layouts with different street continuities (SC = 0.483-1, building-to-street length ratio) are considered. Results show that for the parallel street, the air quality improves in a linear trend as the gap width is increased. However, the relationship between the gap width and air quality is not linear for the perpendicular street. The worst air quality in the street occurs at an intermediate gap width of SC = 0.759, while the air quality is good for both cases when the building gap width is significant (SC = 0.483) or can be neglected (SC = 1, no gaps between buildings). This finding challenges the conventional understanding and highlight the need to reassess the impacts of building gaps on urban air environments.